Problem: documentation/doc/index cannot render
Problem: documentation/doc/index cannot render

Every page is compiled in isolation, so `include::` directives cannot
find any files.

Solution: Make a separate derivation for the content of `doc/index`
that runs in the full `fractalide` source tree, and insert that into
the attributes generated by `loadDir`.

Closes #80

fractalide/fractalide-website-styx,fractalide/fractalide-website-styx,fractalide/fractalide-website-styx